When a general conventional horizontal crankshaft type (or vertical cylinder type) internal combustion engine (hereinafter, an engine HE) was loaded on a vehicle like a lown tractor as shown in FIG. 1, which is provided with a disk mower M under the midsection thereof, a universal joint 4 or a bevel gear unit has been interposed between a horizontal output shaft (a power take-out shaft) 1 of engine HE and a vertical driving shaft 3 of disk mower M.
In the case that engine HE is provided with power transmission endless winding means like a belt or chain wound around the output shaft thereof, or when, as shown in FIGS. 2 or 3, a belt (or sprocket) 2 is wound around an output pulley (or sprocket) 1a on output shaft 1 of engine HE, output pulley 1 (that shown in FIG. 3 is apart from a crankshaft CS) is stressed by a tension F1 of belt (chain) 2 and a rotational torque F2 of itself (or crankshaft CS).
Tension F1 is one way directed (in the cases shown in FIGS. 2 and 3, it is horizontal) and stresses permanently. Rotational torque F2, which is generated during the driving of engine HE (particularly, on starting thereof), mainly causes the vibratory force for engine HE.
The vibratory force mainly caused by rotational torque F2 is vertically large. Therefore, there has been employed such a vibro-isolating supporting structure for engine HE as shown in FIG. 2 or 3, that vibro-isolating members A are interposed between the bottom of engine HE and the upper surface of a frame B serving as a support for engine HE.
In the structure, vibro-isolating member A has vertical and horizontal elasticities, wherein the vertical spring constant thereof is preferably small for absorbing the vertical vibratory force caused by rotational torque F2. In this regard, the flexibility of an elastic material like rubber provided in the vibro-isolating member is so enhanced as to absorb the vibration.
Vibro-isolating member A with a horizontally large spring constant, that is, having the elastic member with a small flexibility can resist against horizontal tension F1 as shown in FIGS. 2 and 3, so that the horizontal contortion of output shaft 1 with engine HE pulled by the belt or chain can be reduced, and the belt or chain can be prevented from slipping or waving.
The conventional power transmission system between the horizontal crankshaft type engine and a vertical driving shaft, such as that of the above-mentioned lown tractor, has required a complicated structure comprising such as a universal joint, bevel gears or the like. If the power for the vertical driving shaft is taken out from a vertical output shaft of a vertical crankshaft type internal combustion engine, the power transmission system therefor will be simplified only with power transmission endless winding means like a belt or chain, as shown in FIG. 4 detailed later, wherein belt 2 is interposed between output pulley 1a on output shaft 1 of an engine VE, serving as a vertical crankshaft type internal combustion engine, and a driving pulley 3a on driving shaft 3 of disk mower M, thereby being reduced in its cost and easing its maintenance.
Conventionally, vertical crankshaft type engine VE has been directly supported by a support such as a frame B as shown in FIG. 5 detailed later, so that the problem has arisen that frame B and engine E are vibrated together in a horizontal rotational direction. If frame B was a part of the vehicle body of the lown tractor shown in FIG. 4, for example, the vehicle body would be vibrated according to the vibration of frame B. Therefore, it has been thought that the engine was supported onto the frame through vibro-isolating means so as to make the frame vibration-proof. In this case, considered location of the vibro-isolating means can prevent the belt or chain from forcible stress.
However, onto the vertical crankshaft type engine is applied mainly horizontal vibratory force caused by the rotation of its crankshaft, so that the vibro-isolating supporting structures shown in FIGS. 2 and 3, which are made resistant against vertical vibration, cannot be used as they are. In this regard, while both forces regarding to the vertical crankshaft type engine comprising the vibratory force caused by the rotary torque and the tension caused by the power transmission endless belt (hereinafter, "belt" may be replaced with "chain".) are horizontal, each of the vibro-isolating members shown in FIGS. 2 and 3, if it was made of elastic material having vertical and horizontal flexibilities, has been located so that a small spring constant direction thereof was set vertical to resist against the vibratory force and a large spring constant direction thereof was set horizontal to resist against the tension, whereby such the large spring constant thereof in the horizontal direction disagrees with that required to make the engine vibro-isolated. Especially, at the position where the directions of the both forces coincide with each other, when the spring constant in thereof parallel to the direction of both forces are set small, or when the flexibility of the elastic material of the vibro-isolating member is set large, the rotational torque is absorbed so as to reduce the vibration, however, the distortion of the engine by the tension of the belt becomes large so as to cause the slipping or waving of the belt, thereby reducing the durability thereof. At the same position, when the spring constant thereof directed in the direction of the both forces is set large, or when the above-mentioned flexibility is set small, the distortion of the engine pulled by the belt is reduced so as to increase the durability of the belt, however, the vibratory force cannot be absorbed, whereby the vibration of the engine becomes large. Thus, the vibro-isolating supporting structure for a vertical crankshaft type engine has included the problems about formation and location of the vibro-isolating member.
Also, while the rotational torque is caused by the rotation of the crankshaft or the output shaft, the fact to be considered for locating the vibro-isolating member is that the rotational vibratory force is generated around the center of gravity of the engine whose weight is entirely unequal.
Furthermore, when the small spring constant direction of the vibro-isolating member coincides with the rotational direction of the vibratory force caused by the rotational torque, the problem has arisen that the deflection of the elastic material, when the engine starts, might exceed the limitation thereof because the rotational torque is considerably large on starting of the engine. To solve the problem, the elastic material of the vibro-isolating member is preferably set to increase its spring constant in the same direction (or to reduce its flexibility) when the deflection thereof exceeds a certain value.